1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter having an impedance that is different between the input and output sides and which has an unbalance-to-balance conversion capability.
2. Description of the Related Art
In recent years, the size and the weight of portable telephones are becoming increasingly smaller. To this end, in addition to the reduction in the number of component parts and the reduction in the size thereof, development of components having a plurality of functions is being pursued.
In view of the above situation, research is being pursued to produce a surface acoustic wave filter having a balance-unbalance conversion capability called balun capability for use in the RF stage of portable telephones, and some surface acoustic wave filters are used mainly in GSM.
In portable telephones, a portion extending from an antenna to a bandpass filter is generally arranged in an unbalanced fashion and has a characteristic impedance of 50 xcexa9. On the other hand, amplifiers or other components following the filter are generally arranged in a balanced manner and have an impedance of 150 xcexa9 to 200 xcexa9. In view of the above, it has been proposed to form a bandpass filter using a surface acoustic wave filter having a capability of converting 50 xcexa9 unbalanced impedance to 150-200 xcexa9 balanced impedance.
In a technique disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, four surface acoustic wave filter elements are used to realize conversion from an unbalanced input to a balanced output. FIG. 28 shows the structure of the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123 cited above. In this surface acoustic wave filter device, a first surface acoustic wave filter unit 203 is constructed by cascading two surface acoustic wave filter elements 201 and 202, and a second surface acoustic wave filter unit 206 is defined by cascading a surface acoustic wave filter element 204 and a surface acoustic wave filter element 205 having a transmission phase characteristic that is different by about 180xc2x0 from that of the surface acoustic wave filter element 204. The input/output terminals of the respective surface acoustic wave filter units 203 and 206 are connected in parallel or in series so that the parallel-connected terminals define unbalanced terminals and the series-connected terminals define balanced terminals.
FIG. 29 illustrates a surface acoustic wave filter device 211 having three IDTs, disclosed in Japanese Unexamined Patent Application Publication No. 6-204781. In this surface acoustic wave filter device 211, two output-side IDTs 212 and 213 are disposed on respective opposite sides such that phases become opposite to each other, and the output terminals of the respective IDTs 212 and 213 define balanced terminals. One end of an input-side IDT 214 disposed at a central location defines an unbalanced terminal. In this structure, the input impedance may be set to 50 xcexa9, and the output impedance may be set to 150-200 xcexa9.
In the surface acoustic wave filters having the balance-unbalance conversion capability, expansion of the passband is desired to meet the requirement of expansion of passbands in portable telephone systems. In surface acoustic wave filters having the balance-unbalance conversion capability, it is required that the transmission characteristics from the unbalanced terminal to the balanced terminals should be equal in amplitude but different by 180xc2x0 in phase, over the passband. That is, an improvement in the degree of balance is desired.
However, in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, the expansion of the passband causes the impedance of the surface acoustic wave filter elements to become capacitive. This resultant capacitance and the parasitic capacitance present between the two cascaded stages cause an impedance mismatch between surface acoustic wave filter units. This makes it difficult to achieve expansion of the passband.
Furthermore, because as many as four surface acoustic wave filter elements are used, complicated interconnections are required, and the complicated interconnections result in an increase in parasitic capacitance which in turn results in degradation in the degree of balance. Furthermore, the use of the large number of surface acoustic wave filter elements results in an increase in size which makes it difficult to obtain a small device size. Furthermore, the use of the large number of surface acoustic wave filter elements results in a reduction in the number of surface acoustic wave filter devices obtained from each wafer, and thus results in an increase in cost.
On the other hand, in the surface acoustic wave filter device disclosed Japanese Unexamined Patent Application Publication No. 6-204781, the two IDTs 212 and 213 have different structures so as to achieve the balanced terminals, and the locations of the IDTs 116 and 117 with respect to the location of the central IDT 214 are different from each other. Such differences in structure or location often cause degradation in the degree of balance. Furthermore, the series connection of the IDTs 212 and 213 on the balanced terminal side results in an increase in loss due to the resistance of the electrode fingers, which results in an increase in insertion loss in the passband.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter device which has unbalanced/balanced input/output terminals and which has a wide passband and a high degree of balance.
In the surface acoustic wave filter device according to a first preferred embodiment, the second and third surface acoustic wave filter elements are arranged such that they are substantially equal in transmission amplitude characteristic within the band but different in transmission phase characteristic by about 180xc2x0, and at least one IDT of the second surface acoustic wave filter element and at least one IDT of the third surface acoustic wave filter element are connected to at least one IDT of the first surface acoustic wave filter element. Thus, the surface acoustic wave filter device can achieve the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the first preferred embodiment of the present invention, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which suppresses degradation in the degree of balance and which makes is easy to expand the passband.
In the surface acoustic wave filter device according to a second preferred embodiment of the present invention, the second and third surface acoustic wave filter elements are arranged such that they are substantially equal in transmission amplitude characteristic within a band but different in transmission phase characteristic by about 180xc2x0, a second IDT of the first surface acoustic wave filter element is connected to the second surface acoustic wave filter element, and a third IDT of the first surface acoustic wave filter element is connected to an IDT of the third surface acoustic wave filter element. Thus, the surface acoustic wave filter device can achieve the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the second preferred embodiment of the present invention, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which suppresses degradation in the degree of balance and which makes is easy to expand the passband.
In the surface acoustic wave filter device according to the second preferred embodiment of the present invention, if the first space and the second space are arranged such that they are different by an amount within the range from about 0.48xcex to about 0.525xcex, the amplitude balance becomes equal to or lower than about 1.5 dB and the phase balance becomes equal to or smaller than about 20xc2x0. As a result, it is ensured that degradation in the degree of balance is prevented.
If the first space and the second space are arranged so as to satisfy Equations 1 and 2, respectively, a sufficiently large bandwidth can be obtained, and degradation in the degree of balance is minimized.
Furthermore, in the surface acoustic wave filter device according to the second preferred embodiment of the present invention, if the first space and the second space are arranged so as to satisfy Equations 3 and 4, respectively, a sufficiently large bandwidth can be obtained and degradation in the degree of balance is minimized, even when a variation in frequency due to a temperature variation is taken into account.
If the first space is within the range from about 1.72xcex to about 1.83xcex and the second space is within the range from about 2.22xcex to about 2.33xcex, the degradation in the degree of balance can be further suppressed and a sufficiently wide bandwidth can be achieved.
In the surface acoustic wave filter device according to the second preferred embodiment of the present invention, if a LiTaO3 substrate made of a LiTaO3 single crystal with an orientation rotated about the X axis from the Y axis to the Z axis within the range from about 36xc2x0 to about 44xc2x0 is used, and if at least one electrode finger is inserted in at least one of the first and second spaces so that the electrode covering ratio of the space in which the electrode finger is inserted becomes equal to or greater than about 50%, propagation of leaky waves becomes dominant, and thus a reduction in the insertion loss is achieved. In particular, if the electrode covering ratio is equal to or greater than about 63%, a further reduction in the insertion loss can be achieved.
In the surface acoustic wave filter device according to the second preferred embodiment of the present invention, if the distance between the first reflector and the second reflector is substantially equal to the distance between the third reflector and the fourth reflector, the filter characteristics of the second and third surface acoustic wave filter elements become substantially equal to each other, and thus, a further suppression in the degradation in the degree of balance is achieved.
The surface acoustic wave filter device according to a third preferred embodiment includes the first, second and third surface acoustic wave filter elements, wherein the second surface acoustic wave filter element is connected to the second IDT of the first surface acoustic wave filter element, the third surface acoustic wave filter element is connected to the third IDT of the first surface acoustic wave filter element, and there is a phase difference of about 180xc2x0 within a passband between the inputs or the outputs of the second IDT and the third IDT of the first surface acoustic wave filter element. Thus, the surface acoustic wave filter device can have the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the first preferred embodiment of the present invention, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which suppresses degradation in the degree of balance and which makes is easy to expand the passband.
In the surface acoustic wave filter device according to the third preferred embodiment of the present invention, if the first space and the second space are arranged such that they are different by an amount within the range from about 0.48xcex to about 0.525xcex, the amplitude balance becomes equal to or lower than about 1.5 dB and the phase balance becomes equal to or smaller than about 20xc2x0. That is, it is ensured that degradation in the degree of balance is prevented.
In the surface acoustic wave filter device according to the third preferred embodiment of the present invention, if the first space and the second space are arranged so as to satisfy Equations 1 and 2, respectively, a sufficiently large bandwidth can be obtained, and degradation in the degree of balance is minimized.
Furthermore, in the third preferred embodiment of the present invention, if the first space and the second space are arranged so as to satisfy Equations 3 and 4, respectively, a sufficiently large bandwidth can be obtained and degradation in the degree of balance is minimized, even when a variation in frequency due to a temperature variation is taken into account.
If the first space is within the range from about 1.72xcex to about 1.88xcex and the second space is set within the range from about 2.22xcex to about 2.33xcex, the degradation in the degree of balance can be further suppressed and a sufficiently wide bandwidth can be achieved.
In the surface acoustic wave filter device according to the third preferred embodiment of the present invention, if the distance between the center of the first IDT and the first reflector and the distance between the center of the first IDT and the second reflector are set to be substantially equal to each other, a further suppression in the degradation in the degree of balance is achieved.
In the surface acoustic wave filter device according to one of the first to third preferred embodiments, if the interdigital overlapping length of the electrode finger of the IDTs defining the first surface acoustic wave filter element is within the range of about 1.5 to about 3.5 times the interdigital overlapping length of the electrode finger of the IDTs defining the second and third surface acoustic wave filter element, degradation in VSWR in the passband is suppressed.
In the surface acoustic wave filter device according to a fourth preferred embodiment of the present invention, the second surface acoustic wave filter element is arranged such that the transmission amplitude characteristic of the second surface acoustic wave filter element is substantially equal to that of the first surface acoustic wave filter element and such that the transmission phase characteristic of the second surface acoustic wave filter element is different by about 180xc2x0 from that of the first surface acoustic wave filter element, and one end of each of the first and second surface acoustic wave filter elements is electrically connected in parallel, and the other end of each of the first and second surface acoustic wave filter elements is electrically connected in series, so that the parallel-connected terminals form unbalanced terminals and the series-connected terminals define balanced terminals. Thus, a balance-unbalance conversion capability is achieved, as in the surface acoustic wave filter device according to the first to third preferred embodiments. Furthermore, because only two surface acoustic wave filter elements are used, further reductions in the size and cost can be achieved.
In the surface acoustic wave filter element according to the fourth preferred embodiment, if the first space and the second space are arranged such that they are different by an amount within the range from about 0.48xcex to about 0.525xcex, the amplitude balance becomes equal to or lower than about 1.5 dB and the phase balance becomes equal to or smaller than about 20xc2x0. That is, it is ensured that degradation in the degree of balance is prevented.
In the surface acoustic wave filter device according to the fourth preferred embodiment of the present invention, if the first space and the second space are arranged so as to satisfy Equations 1 and 2, respectively, a sufficiently large bandwidth can be obtained, and degradation in the degree of balance is minimized.
Furthermore, if the first space and the second space are arranged so as to satisfy Equations 3 and 4, respectively, a sufficiently large bandwidth can be obtained and degradation in the degree of balance is minimized, even when a variation in frequency due to a temperature variation is taken into account.
In the fourth preferred embodiment, if the first space is within the range from about 1.72xcex to about 1.88xcex and the second space is within the range from about 2.22xcex to about 2.33xcex, the degradation in the degree of balance can be further suppressed and a sufficiently wide bandwidth can be achieved.
Also in the fourth preferred embodiment of the present invention, if a LiTaO3 substrate made of a LiTaO3 single crystal with an orientation rotated about the X axis from the Y axis to the Z axis within the range from about 36xc2x0 to about 44xc2x0 is used, and if at least one electrode finger is inserted in at least one of the first and second spaces so that the electrode covering ratio of the space in which the electrode finger is inserted becomes equal to or greater than approximately 50%, propagation of leaky waves becomes dominant, and thus a reduction in the insertion loss is achieved. In particular, if the electrode covering ratio is equal to or greater than about 63%, a further reduction in the insertion loss can be achieved.
In the surface acoustic wave filter device according to the fourth preferred embodiment of the present invention, if the distance between the first reflector and the second reflector is substantially equal to the distance between the third reflector and the fourth reflector, the filter characteristics of the second and third surface acoustic wave filter elements become substantially equal to each other, and thus a further suppression in the degradation in the degree of balance is assured.
In the surface acoustic wave filter device according to the fourth preferred embodiment of the present invention, if the unbalanced-side terminal of the first surface acoustic wave filter element and the unbalanced-side terminal of the second surface acoustic wave filter element are connected to each other via an electrode pattern on the piezoelectric substrate, a reduction in parasitic capacitance is achieved, and thus a further reduction in the insertion loss is achieved.
In a fifth preferred embodiment of the present invention, the surface acoustic wave filter device includes first to third surface acoustic wave filter elements, the first and second spaces are arranged so as to satisfy Equation 1 and 2, respectively, the first IDT defines the unbalanced terminal, and the second and third IDTs are electrically connected in series so as to define the unbalanced terminal. Thus, the surface acoustic wave filter device can have the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the fifth preferred embodiment, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which suppresses degradation in the degree of balance and which makes it easy to expand the passband.
Similarly, also in the surface acoustic wave filter device according to a sixth or seventh preferred embodiment, the surface acoustic wave filter device can have the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the first through sixth preferred embodiments, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which minimizes degradation in the degree of balance and which makes is easy to expand the passband.
The surface acoustic wave filter device according to an eighth preferred embodiment also has the balance-unbalance conversion capability as in the first preferred embodiment. Furthermore, because the particular type of piezoelectric substrate is used as the piezoelectric substrate, propagation of leaky waves becomes dominant, and thus a reduction in the insertion loss is achieved. In particular, if the electrode covering ratio is equal to or greater than about 63%, a further reduction in the insertion loss is achieved. Furthermore, the first and second spaces are arranged so as to be different from each other by about 0.48xcex to about 0.525xcex so that the amplitude balance equal to or less than about 1.5 dB and the phase balance equal to or smaller than about 20xc2x0 can be achieved. Thus, degradation in the degree of balance is prevented. At least one electrode finger is disposed in each area between an electrode finger, which is one of the first IDT""s electrode fingers connected to a signal line and which is disposed at an outermost location, and an electrode finger which is one of the second or third IDT""s electrode fingers connected to a signal line and which is disposed at a location closest to the center, so that the electrode covering ratio in each area descried above becomes equal to or greater than about 50% thereby achieving a further reduction in the insertion loss.
In particular, if the electrode covering ratio is equal to or greater than about 63%, a further reduction in the insertion loss can be achieved.
In the present preferred embodiment, when a series terminal is connected on the unbalanced terminal side, the characteristic in terms of attenuation outside the passband is greatly improved.
Similarly, if surface acoustic wave resonators are connected in series to the respective terminals on the balanced terminal side, the characteristic in terms of attenuation outside the passband is greatly improved.
In the case where a surface acoustic wave filter in the form of a cascaded ladder circuit is disposed on the balanced terminal side, if the ladder-type surface acoustic wave filter is arranged such that its attenuation poles are located near the lower and upper edges of the passband, greater attenuation and better selectivity can be achieved. Furthermore, the surface acoustic wave resonator connected in parallel between the balanced terminals imposes equal influences upon both balanced terminals. This makes it possible to increase the attenuation outside the passband without causing degradation in the degree of balance.
In the surface acoustic wave filter device according to the present preferred embodiment, in the case where a chip on which surface acoustic wave filter elements are disposed is housed in a package, if at least one of the electrode pattern, the package, and the electrical connection member has a structure which are substantially axially symmetric, further suppression in degradation in the degree of balance can be achieved.
In particular, when at least two of the electrode pattern, the package, and the electrical connection member have structures which are substantially axially symmetric with respect to the same symmetry axis, further suppression in degradation in the degree of balance can be achieved.
The surface acoustic wave filter according to various preferred embodiments of the present embodiment can be used in duplexers or communication devices, as described above. In this case, a reduction in the size of the duplexers and communication devices can be achieved.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.